Hyperbaric oxygenation, or hyperbaric oxygen therapy, is a treatment in which an individual is exposed to an environment of increased oxygen at ambient pressure greater than one atmosphere for a predetermined period of time. Hyperbaric oxygen therapy has been approved to treat many conditions, including embolisms, carbon monoxide poisoning, crush injuries, decompression sickness, anemia, and bone infections.
Hyperbaric oxygen therapy involves the application of oxygen (a gas) under pressure. Normal atmospheric pressure exerts approximately 14.7 pounds per square inch (psi), or 760 millimeters of mercury (mm Hg) on skin and on the air that is breathed. This atmospheric air is approximately 79% nitrogen and 21% oxygen, resulting in an oxygen pressure of about 160 mm Hg.
Dalton's law states that the component gas exerts a pressure equivalent to its percentage composition of the mixture. Hyperbaric oxygen therapy is generally discussed using atmospheres absolute (ATA). Normal atmospheric pressure at sea level of 14.7 psi, or 760 mm Hg, is equal to 1 ATA. When diving underwater, water pressure increases by 1 ATA for every 33 feet in depth. Therefore, at 33 feet underwater, an individual will experience 2 ATA of pressure, one ATA from normal atmospheric pressure and one ATA from the addition of 33 feet of water. 2 ATA is equivalent to 29.4 psi.
Normal circumstances of oxygen delivery in the body are dependent on the proportion of oxygen in the air that we breathe, lung function, the amount of hemoglobin in the blood and the body's normal circulation processes (blood pressure). Under normal atmospheric pressure, hemoglobin is approximately 97% saturated with oxygen and there is a smaller amount of oxygen dissolved in the plasma. The hemoglobin molecule is the primary carrier of oxygen to the tissues under normal atmospheric circumstances.
Increasing the inspired oxygen does not improve oxygen delivery by the hemoglobin, and breathing 100% oxygen at normal atmospheric pressure increases the amount of oxygen dissolved in the plasma by a small amount. The amount of oxygen dissolved in the plasma is referred to as the partial pressure of oxygen (pO2).
Between the atmosphere and the mitochondria in the cells is a complicated transport system, along which the partial pressure of oxygen is reduced; this determines the rate at which oxygen can be delivered to the tissues. The succession of diminishing pO2 is called the “Oxygen Cascade.” The oxygen cascade involves a successive decrease in the partial pressure of oxygen as blood flow leaves the lungs and progresses to the cellular level, such that the capillary level and even lower at the intracellular level.
A dramatic increase in the partial pressure of oxygen obtained in the gas breathed in during hyperbaric oxygen therapy has been calculated. A hyperbaric chamber at 2 ATA with 100% oxygen produces two times the 760 mm Hg, or 1,520 mm Hg of oxygen. Breathing air (21% oxygen or 160 mmHg oxygen per ATA) would result in an oxygen partial pressure of 320 mmHg. Hyperbaric oxygen therapy thus provides the ability to dramatically increase the inspired oxygen and thus the amount of dissolved oxygen in the plasma. Most therapeutic applications of HBOT involve 3 ATA (2,280 mmHg of oxygen) or less.
Hyperbaric oxygen therapy has been of particular benefit for treatment of bone infections. Increased diffusion of oxygen from the blood vessels, enhancement of neovascularization (angiogenesis), stimulation of collagen production to build new bone, improvement of blood flow by reduction of edema via vasoconstriction, enhancement of leukocyte ability to kill bacteria, and enhancement of delivery and activity of antibiotics are among the benefits that have resulted from hyperbaric oxygen therapy.
Although treatment of humans using hyperbaric oxygen therapy is known, the therapy may also be useful to healing large animals, such as horses. There exist many differences between horses and humans that make treatment of horses using hyperbaric chambers non-trivial. The horse may be less likely to willingly enter a hyperbaric chamber than a human. Once inside the chamber, the horse is going to continue normal biological functions, such as urinating and defecating, behaviors that are not expected from human subjects. Because the horse may be in the chamber for an extended period of time, the horse may want to drink water. The weight of the horse also complicates treatment. A horse may easily weigh fifteen hundred pounds or more. Getting an animal of such size into a chamber may be problematic for a treatment professional, such as a veterinarian. These non-trivial issues are not simply solved by enlarging a hyperbaric oxygen chamber designed for human use.
Thus, there is a need for a hyperbaric oxygen therapy chamber that may be used to treat large animals, such as horses.